FIELD OF THE INVENTION
In the manufacture of tissue paper and paper towel a web of paper fibers is formed and pressed against a Yankee dryer. After drying, the web is scraped from the surface of the Yankee dryer, giving the web a creped texture which gives the paper it's soft absorptive characteristics. Because only a single dryer is used the Yankee dryer is normally large, typically from twelve to twenty-five feet in diameter. Moreover, the Yankee dryer is heated by steam at pressures of up to 160 psig. A Yankee dryer may be 400 inches long and may have a total weight of over 100 tons. Because of its large size and high operating pressure, a Yankee dryer typically has a cylinder wall thickness of over two inches. Yankee dryers are generally formed from cast iron; a material which has good release characteristics if the surface is properly ground. Thus, the surface of a Yankee dryer requires periodic regrinding to maintain the proper surface finish.
A Yankee dryer is a pressure vessel and the safety precautions typically employed with any pressure vessel or boiler must be observed. In U.S. Pat. No. 4,398,421 an apparatus for measuring the thickness of a work piece which is useful for measuring the thickness of steam boiler walls is disclosed. Determining the wall thickness of a Yankee dryer is useful but it is also desirable to detect small voids within the thickness of the dryer wall.
Existing ultrasonic inspection systems have a limited capability for detecting small voids or finding voids near the surface of cast iron parts. Ultrasonic signals in cast iron are scattered and reflected from grain boundaries present in the cast iron. This characteristic of cast iron makes detecting small discontinuities very difficult. Even measurement of thickness in cast iron can be difficult to perform. One national study has found errors of over 40 percent in thickness measurements of cast iron with some conventional techniques. Ultrasonic transducers typically have an interface zone of up to one-half inch where subsurface discontinuities can not be detected.
X-ray methods are used for the inspection of Yankee dryer rolls. X-ray methods, however, require the use of radioactive sources which are cumbersome and dangerous. In practice, x-ray images are only made of limited portions of the Yankee dryer. Furthermore, x-ray imaging is not effective at detecting hair-line cracks because such cracks do not significantly reduce the density of the material. Hair-line cracks are, however, typically detectable by ultrasonics.
In U.S. application Ser. No. 08/690,763 filed Aug. 1, 1996, now U. S. Pat. No. 5,681,996, which is incorporated herein by reference, a method of ultrasonic inspection was disclosed which is capable of detecting flaws in a Yankee dryer with a high signal-to-noise ratio.
In U.S. Pat. No. 5,681,996 it was disclosed that if an ultrasonic signal is directed at a surface to be inspected at a particular angle, about ten percent of the signal will appear as Harris waves which propagate into the test plate at ninety degrees to the surface. The signal which propagates vertically is polarized, with the result that the signal detects flaws with greatly improved signal-to-noise ratio. The particular angle is between that angle where the ultrasonic signal is refracted so as to propagate parallel to the surface of the metal plate and that angle where the ultrasonic signal is reflected by the test plate. Employing shear waves, for an incident medium of water and a refracting medium of cast iron, the specific angle is approximately 33 degrees from the vertical. For steel the specific angle is approximately 31 degrees. For brass the angle is about 50 degrees.
This particular angle while detecting flaws, produced a signal without an apparent time of flight--meaning that the depth of the flaw could not be determined with the disclosed method. Methods of finding the depth of a flaw using ultrasonic energy can be difficult to calibrate. Traditional methods require a flat bottomed hole drilled from a surface opposite the surface from which the ultrasonic interrogation is conducted. A hole with a flat bottom is difficult to drill and difficult to precisely line up parallel to the interrogation surface.
In U.S. application Ser. No. 08/948,959, filed Oct. 10, 1997, which is incorporated herein by reference, a method of ultrasonic inspection was disclosed which is capable of determining the depth of any detected flaw in a Yankee dryer.
In the '959 application, an ultrasonic inspection method and apparatus based on a calibration method and an empirically observed interrogation angle was described. An inspection method for a cast iron dryer shell which can reliably detect discontinuities and their locations within the casting was described. The U.S. Pat. No. 5,681,996 patent and the Ser. No. 08/948,959 application relate to ultrasonic instruments for nondestructive testing of materials in general and for nondestructuring testing of large cast cylinders in particular. The invention of the subject application is related to the ultrasonic interrogation of coated materials in general and for nondestructive testing of coated, large cast cylinders in particular.
The outside shell or surface of paper and tissue machine cast iron dryer cylinders becomes worn in service due to abrasion during operation. The primary cause of wear to the outside shell of a paper machine dryer cylinder is attributable to a doctor blade.
A doctor blade is generally brought into intimate contact with the outside shell or surface of a dryer cylinder in order to scrape or crepe off the web of paper or tissue traveling over the dryer cylinder.
Outside diameter shell or surface wear to a cast iron dryer cylinder adversely affects the quality of the paper or tissue product being formed. Replacing entire worn dryer cylinders is extremely costly. So that the entire dryer cylinder does not have to be replaced when the cylinder's outside shell becomes worn, the cast iron dryer cylinder outside surface is often machined or ground down until a smooth surface is obtained and a coating of metalspray is applied to the dryer cylinder surface such that the dryer cylinder is capable of further use.
There are a number of metalspray compositions capable of being used for large cast iron dryer shells generally known to those skilled in the art. Metal spray application is also generally known to those skilled in the art. Essentially, molten metal is sprayed onto the surface or outer shell of a large cast iron dryer. Typically, at the outer edges of the metalspray flame, cold splatter, known to those skilled in the art, may be deposited onto the surface of the drying cylinder. The larger the arc of the metalspray, the more cold splatter that is deposited. Cold splatter is a miniature disbond of metalspray material between the metalspray coating and the substrate upon which the coating is applied. Detectable quantities of cold splatter cause problems related to heat transfer, surface quality, and other problems as can be appreciated by those skilled in the art. Such problems compromise the quality of the tissue or paper product being formed.
Typically, during clean-out procedures of a paper or tissue machine dryer section, dryer cylinders are often exposed to water streams. Although dryer cylinders are not usually purposefully exposed to water streams, such exposure does occur. If the outside shell or surface of a dryer cylinder has been metalsprayed, there will be a difference in the coefficient of thermal expansion between the metalspray coating and the cast iron substrate. When water contacts the metalsprayed shell, the shell and metalspray begin to cool. The metalspray will cool at one rate and the cast iron dryer cylinder will cool at another rate. The difference in the thermal coefficient between the two materials can cause cracking or disbonding of the metalspray. If the metalspray is not properly adhered to the surface of the cast iron cylinder, the metalspray will likely fall or wear off at the location of the poor adherence during a paper or tissue making operation. If the metalspray wears or falls off, problems associated with poor heat transfer and surface quality will occur and a poor tissue or paper product will result.
What is needed is a method for performing complete inspection of a coated cast iron dryer cylinder for quality control and assurance of thin metalspray adhesion to cast iron dryer shells.